In a recent pilot study, we measured long-term pulmonary clearance in a small group of smokers and non-smokers; a one-time inhalation of magnetic dust was used as a harmless tracer. This dust was cleared much slower in smokers than in non-smokers; after one year, five times as much dust was retained in the lungs of smokers as in non-smokers. This higher retention may be one cause of the high rate of pulmonary disease experienced by smokers. We propose to further investigate this result with a larger three-part study. In the first part, a magnetic tracer dust will again be inhaled, and clearance will be measured with more elaborate techniques; six heavy and three ex-heavy smokers (of high tar-nicotine cigarettes), three heavy smokers (low tar-nicotin), and three non-smokers will be studied. Our purpose is to determine if this high retention (and attendant risk) is a function of nicotine-tar content, and if it declines to non-smoker levels in ex-smokers. In the second part, magnetic dust content will be measured in the lungs of ten heavy smokers and ten non-smokers, all with long exposure to urban dust. Also, the same measurements will be made of ten smoking and ten non-smoking arc welders. The purpose is to determine if the higher dust retention seen in smokers also occurs in individuals experiencing normal urban living or during occupational exposure. In the third part, the dust content from sections of autopsied lungs will be measured both magnetically and chemically. At least 150 cases divided into smokers and non-smokers, and including many urban dwellers, will be studied. The purpose is to correlate the amount of magnetic dust to total dust in the lung, and to lung pathology. This will determine if magnetic measurements are a good index of total retained dust, and of lung pathology.